Known from EP-A-0 300 536 is the culture of plants on substrate that is formed by rock-wool.
This literature reference further describes a system for supplying water, optionally mixed with nutrients, to the substrate in order to foster the best possible growth of the plant. It is usual to supply an excess of water.
There are various reasons herefor, such as: to prevent the accumulation of ballast salts, to compensate for flow differences in feed members for water, to compensate for differences in the water consumption of different plants, and to restore the correct ratio of the (chemical) elements supplied as nutrients.
It should be noted here that according to the system as described in EP-A-0 300 536 it is possible to considerably limit the excess of water, since using measurement of the degree of moistness of the rock-wool substrate it is possible to determine a reasonable estimate of the water requirement of the plants. It will nevertheless be necessary to discharge an excess of water.
In order to discharge this excess water a drainage system is arranged which in accordance with the cited literature reference is formed by a number of thin hoses extending into the rock-wool substrate and connected onto a drainage pipe with a larger diameter. A certain underpressure is maintained in this drainage pipe so that, partly depending on the capillary properties of the drainage hoses, the excess water is discharged from the rock-wool substrate.
This known system suffers from the drawback that the roots of the plants that are cultivated in the substrate have the tendency to grow to the drainage hoses and even into the drainage hoses, so that these are closed off and a good action is no longer achieved.
There is the further danger that the drainage hoses, which are in general simply inserted into the substrate, are pulled out of their proper position. Since these drainage hoses are only simply inserted into the rock-wool substrate, they can be easily pulled out of their proper position and then no longer fulfil their proper function or can simply be pulled entirely from the rock-wool substrate.